Colorbox doesn't respect video size inside iframe and it's needed additional CSS/Javascript to fit iframe in colorbox popup without scrollbar.
I will attach a patch for that.
Needs review
2.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
It seems to work for me.
I am not sure that would work in all cases, if someone had other fields in the media and want to display them inside the modal, it might be more appropriate to be larger than the iframe sometimes.